Semiconductor and other similar industries, often use optical metrology equipment to provide non-contact evaluation of substrates during processing. One type of optical metrology device is an interferometer which superimposes electromagnetic waves, e.g., light, reflected from a reference surface and a sample surface resulting in interference from which information about the sample may be extracted. Interferometers are capable of measuring small height differences on a sample. Interferometers, for example, are sometimes used to detect defects, e.g., by comparing a measured height to reference height, where measured height that is much greater or lesser than the reference height may indicate a defect. Other optical metrology devices may measure characteristics of a sample from light that is reflected off of the surface of the sample.
Interferometry and many other optical metrology techniques rely on the reflection of light from the surface of the sample. Semiconductor processing, however, often uses layers that are at least partially transparent to specific wavelengths of light. An accurate measurement of a sample with a layer that is at least partially transparent to the wavelengths of light used by the optical metrology device is difficult.